The present invention relates to a cylindrical-surface seal used as one kind of a shaft seal device.
A cylindrical-surface seal has been heretofore well known which has a construction as shown in FIGS. 5 to 11.
The cylindrical-surface seal 1A shown in FIGS. 5 to 8 is called a 2-ring type which has a housing 2 encasing therein a seal ring 4, a cover ring 5, a key 6, and an element 3 in combination of an extension spring 7 and a compression spring 8, and a spring retainer 9 is placed as a lid which is fastened by a snap ring 10. The seal ring 4 and the cover ring 5 are mainly formed of carbon material, both the rings 4 and 5 being equally divided into three sections or more according to the size of a shaft 11 and combined with a divisional phase deviated in a circumferential direction. A divided portion of the seal ring 4 comprises a step joint having a lapping surface 12a which extends vertically with respect to an axial direction and in a circumferential direction in order to stop an axial leak path. The seal element 3 is stopped through the key 6 by means of a rotation lock pin 13 extending upright on the housing 2, and is fastened so as to be urged toward a shaft 11 by means of the extension spring 7 so that the seal element 3 may follow the radial movement of the rotating shaft 11 and wear of the inner diameter surface of the seal ring 4 in sliding contact with the outer diameter surface of a runner 14 disposed externally of the shaft 11.
A seal mechanism of the cylindrical-surface seal 1A of the 2-ring type described above will be explained hereinafter.
The radial leak path resulting from the division of the seal ring 4 in the circumferential direction is closed by mutual contact of the lapping surfaces 12a of the step joint as previously described. The diametral leak path in the step joint portion is closed by the contact of the cover ring 5 different in phase cf the divided portion from the seal ring 4 so as to cover the step joint from the outer diameter side thereof. The inner diameter surface of the seal ring 4 in sliding contact with the outer diameter surface of the runner 14 which rotates integral with the shaft 11 is formed so as to have the same diameter with a seal dam portion 15 in sliding contact with the outer diameter surface of the runner 14 in a closed state to form a primary seal portion and a preformed bearing pad portion 16 which shares a contact surface pressure with the outer diameter surface of the runner 14 with the seal dam portion 15 to receive the pressure, the bearing pad portion 16 having a pressure balance groove 17 for introducing a fluid on the high pressure side P1 in order to relieve a radial load caused by fluid pressure to the sliding contact portion including said primary seal, the bearing pad portion 16 does not provide a primary sealing function. The seal element 3 is axially pressed by the compression spring 8 whose one end is supported by the spring retainer 9 on the high pressure side P1, and an end surface 18 opposite to the compression spring 8 is in con-act with an end wall of the opposed housing 2 to form a static secondary seal. Reference numeral 19 designates a pressure balance groove for introducing a fluid on the high pressure side P1, the pressure balance groove being formed in the aforesaid end surface 18 in order to relieve the axial load caused by the fluid pressure to the contact portion between the end surface 18 including the second seal portion and the end wall of the housing 2.
Next, a cylindrical-surface seal lB shown in FIGS. 9 and 10 is called a 3-ring type. A seal element 3 is formed in its inner diameter surface with a pressure introducing groove 17' which is divided in the same phase as that of the cover ring 5 to communicate a pressure balance groove 17 of the inner diameter surface of the seal ring 4 with a space on the high pressure side P1 and has a back ring 21 fastened by an extension spring 20 separately from the extension spring 7 adapted to fasten the seal ring 4 through the cover ring 5, the seal ring 4 and the cover ring 5 being axially pressed by the compression spring 8 through the back spring 21. The divided portion of the seal ring 4 is not formed with the step joint as in the above-described cylindrical-surface seal 1A of the 2-ring type but cut in an axially straight plane 12b. The radial leak path in the divided portion of the seal ring 4 is closed by the cover ring 5 similarly to the aforesaid 2-ring type whilst the axial leak path is closed by the back ring 21 which is different in the divisional phase from the seal ring 4 but has the same phase as that of the cover ring 5.
A cylindrical-surface seal 1C shown in FIG. 11 is called a 1-ring type. A seal ring 4 is fastened by an extension spring 7 directly without interposition of a cover ring. The divided portion of the seal ring 4 comprises a box joint which closes an axial leak path in a lapping surface 12c vertical to the axial direction and closes a radial leak path in a lapping surface 12d parallel with the outer diameter surface and inner diameter surface of the seal ring 4.
The above-described cylindrical-surface seals 1A, 1B and 1C provide a good dynamic seal by the provision of a completely closed state between the seal dam portion 15 of the inner diameter surface of the seal ring 4 and the outer diameter surface of the runner 14 on the shaft 11 side. However, actually, there sometimes occurs an insufficient contact between the seal dam portion 15 and the runner 14 due to an error in working accuracy of the inner diameter surface of the seal ring 4, resulting in an excessive amount of leak which poses a practical problem. The imperfect contact of the seal dam portion 15 occurs in the case where the inner diameter surface of the seal ring 4 is worked into a tapered shape so as to be slightly larger in diameter on the side of the seal dam portion 15 or the outer diameter surface of the runner 14 is worked into a tapered shape to be slightly smaller in diameter at a portion corresponding to the seal dam portion 15. At the time when assembled after being manufactured but not yet used, the inner diameter surface of the seal ring 4 as well as the outer diameter surface of the runner 14 on the mating side are in the state not finished by machining. Accordingly, there is unavoidably present a fine clearance between the contact surfaces of both the elements 4 and 14. The unevenness of this clearance comprises one of significant causes for the initial leak.
If the inner diameter surface of the seal ring 4 becomes run in the outer diameter surface of the runner 14 due to the wear resulting from the sliding contact, the seal dam portion 15 will be in the stabilized contact state over the whole periphery. Accordingly, the aforementioned imperfect contact state is no longer a problem. However, the seal ring 4 is originally formed of material which is resistant to wear, and in addition, the load caused by fluid pressure acting in a radial direction is relieved by formation of the pressure balance groove 17. Thus, the seal ring 4 does not readily wear. Therefore, it takes a considerable time (generally, 100 to 200 hours) till the seal dam portion 15 becomes completely run in on the runner 14 due to wear to exhibit a good sealing property, during which period a large amount of leaks unavoidably occurs.
Accordingly, in order to eliminate such an inconvenience, the working accuracy (cylindricity) of the seal ring 4 and the runner 14 should be increased to an extremely high level. However, in fact, it is difficult to perform working with high accuracy as described.